In recent years there has been a flurry of activity aimed at evaluating the land use consequences of biofuels programs and the associated carbon releases. In this paper we argue that these studies have tended to underestimate the ensuing land use change, because they have ignored the role of irrigation, and associated constraints on cropland expansion. In this paper, we develop a new general equilibrium model which distinguishes irrigated and rainfed cropping industries at a global scale. Using the new model we evaluate the implications of land use change due to US ethanol programs, in the context of short run constraints on the expansion of irrigated cropland. Since irrigated area tends to offer a higher yield than its rainfed counterpart, this provides an upper bound on the change in cropland following biofuel expansion. We find that the biofuel-induced expansion in global cropland cover is about 16 percent larger when the irrigation constraint is imposed. This translates into a 21 percent increase in land use emissions due to US ethanol production. This estimate represents an upper bound, since irrigated area can be expanded over the medium run in many places around the world.